1. Field of the Invention
The present invention relates to a faulty sewing detector which, in performing an over-edge chain stitch that is made usually by an overlock sewing machine, detects a faulty sewing that one of two overlapping fabrics to be sewed deviates from an other, and informs it to the operator of this condition.
2. Description of the Prior Art
Faulty sewing that might occur in performing an over-edge chain stitch, etc. with an overlook sewing machine is generally classified as so-called slip out and twisting. specifically, as shown in FIG. 6, the slip out means that, of two overlapping fabrics sewed 1 and 2, the lower fabric 2 that is difficult to view by the operator deviates in its width direction X orthogonal to a sewing direction Y, with respect to the upper fabric 1, thereby a side end 2a of the lower fabric 2 departs from a seam 3. As shown in FIG. 7, the twisting means that sewing finish ends 1b and 2b of two fabrics 1 and 2, respectively, do not match with each other to cause twisting in a sewing direction Y. In the following description, twisting in direction X and that in Y direction are referred to as "slip out" and "twisting", respectively, and a state in which fabrics 1 and 2 are not matched, irrespective of X and Y directions, is referred to as "fabric twisting."
Referring to the faulty sewing called as twisting, as shown in FIG. 7, the sewing finish ends 1b and 2b can be easily held within a permissible twisting range making such a correction where the operator moves one of the fabrics 1 or 2. On the other hand, as to the faulty sewing called as slip out, as shown in FIG. 6, correction in the course of sewing is very difficult because twisting of the lower fabric 2 is difficult of view. In addition, the slip out is fatal to a sewing product and thus it is important to reliably remove it as defective article, at the sewing stage. This can be achieved by a faulty sewing detector.
As a faulty sewing detector of this type, there are known, for example, ones which are disclosed in Japanese Utility Model Examined Publication No. 59-25349 (1984) and Japanese Patent Unexamined Publication No. 3-91 (1991)
A device as described in the former publication detects a faulty sewing due to slip out by detecting the change in the transmitted light quantity which corresponds to the change in the number of overlap of fabrics sewed, with a sensor of the light transmission type which comprises a projector for projecting an inspection light beam to the fabrics, and a receptor for receiving the light beam passed through the fabrics.
A device as described in the latter publication comprises a first sensor for detecting the number of overlap of fabrics sewed, and a second sensor for detecting the presence of the fabric, which is spaced, with respect to the first sensor, a distance corresponding to a permissible slip out in the sewing direction. Thereby, a faulty sewing due to slip out or twisting is detected in a combination of the detecting actions of the first and second sensors.
The conventional faulty sewing detecting devices as described can perform the respective predetermined sewing fault detection. In these devices, however, each sensor is disposed at a position which corresponds to the fabric sewing area of a needle, and, in front of a needle location in the sewing direction. Thus, fabric twisting detected by such sensors is one which is already present prior to sewing, Namely, it is a forecast detection that, if sewing proceeds in this state, a sewing article may have a faulty sewing. With this forecast detection, when a fabric has no fabric twisting in front of a needle location, but has a fabric twisting until it reaches the needle location, a faulty sewing due to the fabric twisting cannot be detected. In performing a curve stitch which is, in particular, liable to involve slip out when a fabric sewed is fed from a position in front of a needle location to the needle location, it is impossible to detect a faulty sewing due to the slip out.
More specifically, when a fabric sewed is, for example, a T shirt or underwear as shown in FIG. 8 or 9, a body fabric 1 and a sleeve fabric 2 are fed to a needle location in a state in which an in-curve 1c of the body fabric 1 and an out-curve 2c of the sleeve fabric 2 are overlapped and matched with each other, as shown by the dotted line in FIG. 9. Then, sewing proceeds such that a seam 3 is formed along a curve part in which the in-curve 1c conforms to the out-curve 2c. In this case, the operator overlaps the in-curve 1c of the body fabric 1 and the out-curve 2c of the sleeve fabric 2 as perfect as possible, and feeds them in front of the needle location. However, since the part to be sewed is a curve, the in-curve 1c and the out-curve 2c are often not matched at the time when they reach the needle location by the succeeding feed operation. Thus, the conventional faulty sewing detecting device cannot detect the faulty sewing due to the slip out as described. Hence it is required to examine the article after the sewing therefor is terminated.
Further, in the conventional faulty sewing detecting device, since each sensor is disposed in front of the needle location, it is difficult to allow space in front of the needle location, available for various attachments to assist sewing operation. Alternatively, the function of detecting a predetermined faulty sewing is liable to be impaired by installation of attachments.